The present invention relates to reflective mixed-mode twisted nematic liquid crystal displays, and mote particularly to such displays with an optimised twist angle, polariser angle and retardation.
Reflective-mode liquid crystal displays (LCDs) are known to offer various advantages compared with transmissive-mode liquid crystal displays. Reflective-mode liquid crystal displays have a lower power consumption, are more easily readable in sunlight, have a thinner profile and weigh less.
Transmissive-mode matrix liquid crystal displays with back-light illumination commonly use a 90xc2x0 twisted nematic cell. Such cells work by the mechanism of polarisation rotation to provide the electro-optic effect. An incoming linearly polarised light follows the twist of the LC directors. Therefore, when light has traversed the cell, its polarisation axis is rotated by approximately 90xc2x0 , and the light is transmitted by a crossed polariser.
When such transmissive-mode twisted nematic cells are employed in reflective displays, poor brightness and low resolution result. The poor brightness is caused by the light passing through the crossed polariser twice, and the low resolution is caused from the parallax which is due to the thickness of the polariser sitting between the liquid crystal cell and the reflecting mirror.
Shin-Tson Wu et al describe in the paper xe2x80x9cMixed-Mode Twisted Nematic Liquid Crystal Cells for Reflective Displaysxe2x80x9d, Applied Physics Letters 68(11), Mar. 11, 1996, pages 1455-1457, an improved reflective-mode display called a mixed-mode twisted nematic cell. Although the arrangement proposed by Wu et al is advantageous in that it improves the brightness and eliminates the parallax caused by using a transmissive twisted nematic cell, the arrangement proposed does not provide the optimum performance.
The present invention is concerned with improving the performance of reflective mixed-mode twisted nematic liquid crystal displays.
According to the present invention there is provided a reflective mixed-mode twisted nematic liquid crystal display having a twist angle of between 67xc2x0 and 83xc2x0, a polariser angle of between 5xc2x0 and 111xc2x0, and a retardation of between 0.18 and 0.28.
The polariser angle may suitably comprise a plurality of ranges suitably from 5xc2x0 to 21xc2x0 and from 95xc2x0 to 111xc2x0.
In one arrangement the twist angle is 76xc2x0xc2x13xc2x0, the polarised angle is 18xc2x0xc2x13xc2x0 and the retardation is 0.25xc2x10.03. In this arrangement, preferably, the twist angle is 76xc2x0, the polariser angle is 18xc2x0 and the retardation is 0.25.
In another arrangement, the twist angle is 70xc2x0xc2x13xc2x0, the polariser angle is 8xc2x0xc2x13xc2x0 and the retardation is 0.21xc2x0xc2x10.03. In this arrangement, preferably, the twist angle is 70xc2x0, the polariser angle is 8xc2x0 and the retardation is 0.21.
In a further arrangement, the twist angle is 80xc2x0xc2x13xc2x0, the polariser angle is 16xc2x0xc2x13xc2x0 and the retardation is 0.23 xc2x10.03. In this arrangement, preferably, the twist angle is 80xc2x0, the polariser angle is 16xc2x0 and the retardation is 0.23.
The polariser angle may be 98xc2x0, 108xc2x0 or 106xc2x0, each xc2x13xc2x0. In each case the retardations may respectively be 0.21, 0.25 and 0.23. The optical performance of devices incorporating such polariser angles is similar to or identical to that rotation of the polariser through the above values minus 90xc2x0, namely on 8xc2x0, 18xc2x0 or 16xc2x0 polariser angle.
The display may comprise a polariser, a liquid crystal cell and reflecting means, and may be a direct-view display.
Alternatively, the display may comprise a polarising beam splitter, a liquid crystal cell and reflecting means. This type of display is usually a projection display.